feat: sub-routers and route prefix composition (include_router) #46

Context

• Today App is a flat registry: every route is registered on one object. Large APIs need namespaced paths (/api/v1/..., /admin/...) without repeating prefixes on every decorator.
• See Feature gaps.

Direction

• Add something like APIRouter / include_router(router, prefix=..., **defaults) in Python, merging route tables and OpenAPI paths into the parent App before freeze().
• Alternatively: a prefix argument on the existing get/post/... decorators only (weaker, but less API surface).
• OpenAPI: merged paths must remain valid; duplicate path+method should error at register time.

Acceptance criteria

• At least one example under examples/ with two routers and a shared prefix.
• Pytest: routes under prefix resolve; OpenAPI lists prefixed paths.
• Document in docs/routing.md.

Research

• FastAPI APIRouter; Starlette Router / mounts.